Method, device and apparatus for measuring the concentration of creatinine, and method, device and apparatus for measuring the amount of salt using the same

ABSTRACT

A method for measuring a concentration of creatinine includes the steps of: (A) mixing a sample containing creatinine with a creatinine quantitative reagent containing a metal complex of at least one of hexacyanoferrate and hexacyanoruthenate in the absence of picric acid and any enzyme responsive to creatinine, to cause the creatinine to reduce the metal complex; (B) electrochemically or optically measuring the amount of the metal complex reduced in the step (A); and (C) determining the concentration of the creatinine contained in the sample from the amount of the reduced metal complex measured in the step (B).

TECHNICAL FIELD

The invention relates to a measurement method, device, and apparatus forquantifying creatinine or salt contained in a sample.

BACKGROUND ART

The measurement of the concentration of creatinine contained in a sampleis important in the fields of clinical chemistry and analyticalchemistry. Since creatinine is a product of the endogenous metabolism ofmuscle, it is known that the amount of creatinine in urine reflectstotal muscle mass. Hence, it is believed that the amount of creatinineexcretion in the urine of each individual in a day is usually constantand does not vary from day to day. As such, the amount of urinarycreatinine may be used as a measure of the thickness of excreted urine.Also, the amount of creatinine in urine and blood increases/decreasesdue to uremia or decreased renal function. Thus, the measurement of theamount of creatinine in urine or blood permits determination of thepresence or absence of uremia or decreased renal function.

A known method for measuring creatinine concentration is a method basedon Jaffe reaction using an alkaline picrate solution. According to thismethod, the orange-red product formed by the reaction between picricacid and creatinine is spectroscopically measured (see, for example, PTL1).

Another known method for measuring creatinine concentration is a methodusing an enzyme that reacts specifically with creatinine. An example ofsuch an enzymatic method is a method of decomposing creatinine usingcreatinine deiminase (see, for example, PTL 2). According to thismethod, the amount of ammonia produced by the decomposition ofcreatinine is measured based on the change in pH, potential, or the liketo determine creatinine concentration.

Another enzymatic method is a method of measuring creatinineconcentration by carrying out the following reactions of formulas (1) to(3).

Creatinine+Water→Creatine   (1)

Creatine+Water→Sarcosine+Urea   (2)

Sarcosine+Water+Oxygen→Glycine+Formaldehyde+Hydrogen Peroxide   (3)

The enzymes used to catalyze the reactions of formulas (1) to (3) arecreatinine amidohydrolase (creatininase), creatine amidinohydrolase(creatinase), and sarcosine oxidase or sarcosine dehydrogenase,respectively. Creatinine is quantified, for example, by a method ofusing a leuco pigment and the Trinder reagent together with a peroxidaseto cause the hydrogen peroxide produced in formula (3) to give a colorfor spectroscopic quantification (see, for example, PTL 3). Also,another creatinine quantification method is a method ofelectrochemically oxidizing the hydrogen peroxide produced in formula(3) at an electrode to cause a current to flow and quantifyingcreatinine from the current (see, for example, PTLs 4 and 5).

Further, still another enzymatic method is a method of quantifyingcreatinine by carrying out the reactions of formulas (1) and (2) andadditionally carrying out the reaction between sarcosine and an electronmediator (mediator) instead of the reaction of formula (3) (see, forexample, PTLs 6 and 7).

PTL 6 discloses a creatinine biosensor comprising at least a pair of aworking electrode and a counter electrode on a substrate, wherein areagent solution is dried on the electrodes or on the substrate near theelectrodes to immobilize the reagent. The reagent solution is preparedby dissolving creatininase, creatinase, sarcosine oxidase, and potassiumferricyanide (mediator) in a buffer solution of pH 7 to 8.5. It is alsodisclosed that a buffer solution pH of less than 7 or greater than 8.5is not preferable since the enzyme activity decreases.

PTL 7 discloses quantifying creatinine by colorimetry or anelectrochemical detection method using sarcosine oxidase and a mediatorencapsulated in cyclodextrin. Specifically, PTL 7 cites α-naphthoquinone(1,4-naphthoquinone) as an example of a mediator encapsulated incyclodextrin, but discloses that mediators are not suitable for theenzymatic quantification of creatinine if they are not encapsulated incyclodextrin.

Also, still another enzymatic method is a method of spectroscopicallyquantifying creatinine by carrying out the reactions of formulas (1) and(2) and additionally carrying out the reaction between sarcosine and atetrazolium indicator instead of the reaction of formula (3) (see, forexample, PTL 8). PTL 8 discloses that the reagent composition forcreatinine quantification comprises a reagent mixture composed ofcreatinine hydrolase, creatine amidinohydrolase, sarcosinedehydrogenase, thiazolyl blue serving as the tetrazolium indicator, andpotassium phosphate of pH 7.5.

Also, still another enzymatic method is a method of convertingcreatinine to glycine and formaldehyde by use of creatinineamidohydrolase, creatine amidinohydrolase, and sarcosine dehydrogenase,causing the produced formaldehyde to give a color with the aid of acolor reagent, and quantifying creatinine from the absorbance (see, forexample, PTL 9). PTL 9 discloses using a phosphate buffer solution of pH7.5 and potassium ferricyanide serving as a reaction accelerator forpromoting the formation of formaldehyde in addition to creatinineamidohydrolase, creatine amidinohydrolase, sarcosine dehydrogenase, andthe color reagent.

Also, still another enzymatic method is a method of quantifyingcreatinine using an electrode on which a polymer that catalyzes thehydrolysis of creatinine, sarcosine oxidase, and a mediator areimmobilized (see, for example, PTL 10). PTL 10 discloses using, forexample, potassium ferricyanide, ferrocene, an osmium derivative, orphenazine methosulfate (PMS) as the mediator.

Still another creatinine quantification method is a method using1,4-naphthoquinone-2-potassium sulfonate (see, for example, PTL 11 andNPLs 1 to 3).

Citation List Patent Literature

PTL 1: U.S. Pat. No. 3,705,013

PTL 2: Japanese Laid-Open Patent Publication No. 2001-512692

PTL 3: Japanese Laid-Open Patent Publication No. Sho 62-257400

PTL 4: Japanese Laid-Open Patent Publication No. 2003-533679

PTL 5: U.S. Pat. No. 5,466,575

PTL 6: Japanese Laid-Open Patent Publication No. 2006-349412

PTL 7: Japanese Laid-Open Patent Publication No. 2005-118014

PTL 8: Japanese Laid-Open Patent Publication No. Sho 55-023998

PTL 9: Japanese Laid-Open Patent Publication No. Sho 54-151095

PTL 10: Japanese Laid-Open Patent Publication No. 2003-326172

PTL 11: Japanese Laid-Open Patent Publication No. Sho 63-033661

Non Patent Literature

NPL 1: Sullivan et al., “A Highly Specific Test for Creatinine”, Journalof Biological Chemistry, 1958, Vol. 233, No. 2, p. 530-534

NPL 2: Narayanan et al., “Creatinine: A Review”, Clinical Chemistry,1980, Vol. 26, No. 8, p. 1119-1126

NPL 3: Cooper et al., “An Evaluation of Four Methods of MeasuringUrinary Creatinine”, 1961, Vol. 7, No. 6, P. 665-673

SUMMARY OF INVENTION Technical Problem

However, the above-described conventional methods have the followingproblems.

In the method described in PTL 1, due to the influence of interferentssuch as amino acids including glycine, histidine, glutamine, and serine,proteins, sugars such as glucose, acetone, and bilirubin, it isdifficult to accurately quantify creatinine in samples containing suchsubstances, for example, in biological samples such as urine and blood.For example, amino acids and sugars such as glucose undesirably reactwith picric acid.

Also, in the method described in PTL 2, it is difficult to accuratelyquantify creatinine since the change in pH or potential is unstable.

Also, in the methods described in PTLs 2 to 10, if a sample contains anion species such as salt or urea, the enzyme activity decreases due toenzyme denaturation. Thus, the reaction speed varies with theconcentration of the ion species or urea contained in the sample.Therefore, in the quantification of creatinine in a sample containing anion species or urea, for example, a biological sample such as urine orblood, the measurement result involves an error depending on theconcentration of the ion species or urea contained in the sample.

Further, with respect to the methods of PTL 11 and NPL 1 using1,4-naphthoquinone-2-potassium sulfonate, NPLs 2 and 3 have reportedthat the reproducibility of the results measured by these methods isvery low.

In view of the above-described problems associated with conventionalart, it is therefore a first object of the invention to provide acreatinine concentration measuring method, device and apparatus capableof quantifying creatinine contained in a sample with good accuracy andgood reproducibility.

It is a second object of the invention to provide a salt measuringmethod, device and apparatus capable of quantifying the amount of saltcontained in urine with good accuracy and good reproducibility.

Solution to Problem

In order to solve the above-noted problems with conventional art, themethod for measuring the concentration of creatinine according to theinvention includes the steps of:

(A) mixing a sample containing creatinine with a creatinine quantitativereagent containing a metal complex of at least one of hexacyanoferrateand hexacyanoruthenate in the absence of picric acid and any enzymeresponsive to creatinine, to cause the creatinine to reduce the metalcomplex;

(B) electrochemically or optically measuring the amount of the metalcomplex reduced in the step (A); and

(C) determining the concentration of the creatinine contained in thesample from the amount of the reduced metal complex measured in the step(B).

The method for measuring the amount of salt according to the inventionincludes the steps of:

(a) mixing urine, which is a sample, with a creatinine quantitativereagent containing a metal complex of at least one of hexacyanoferrateand hexacyanoruthenate in the absence of picric acid and any enzymeresponsive to creatinine, to cause creatinine contained in the urine toreduce the metal complex;

(b) electrochemically or optically measuring the amount of the metalcomplex reduced in the step (a);

(c) measuring an electrical property of the urine; and

(d) determining a value reflecting the amount of salt excreted in theurine from the amount of the reduced metal complex measured in the step(b) and the electrical property measured in the step (c).

It is preferable to perform the step (c) before the step (a), or afterthe step (b) and before the step (d).

In the step (A) and step (a), the pH of the sample after the mixing ispreferably set to 2.5 or more and 7 or less, and more preferably set to3 or more and 6 or less. In the step (A) and step (a), the sample ispreferably mixed with a phosphate buffer; in this case, it isparticularly preferable to adjust the pH of the sample to 5 to 6. In thestep (A) and step (a), if the sample is mixed with a cationichydrophilic polymer, the reproducibility of the reaction betweencreatinine and the reagent improves. The cationic hydrophilic polymer ispreferably cationic guar gum.

The device for measuring the concentration of creatinine according tothe invention is a device used in the above-mentioned method formeasuring the concentration of creatinine. This device comprises:

a sample holding space for holding a sample containing creatinine in theabsence of picric acid and any enzyme responsive to creatinine;

a sample inlet for introducing the sample into the sample holding space,the sample inlet communicating with the sample holding space;

a creatinine quantitative reagent disposed in the sample holding space,the creatinine quantitative reagent containing a metal complex of atleast one of hexacyanoferrate and hexacyanoruthenate; and

two or more electrodes disposed in the sample holding space or anoptical measurement window disposed on the sample holding space.

Also, the device for measuring the amount of salt according to theinvention is a device used in the above-mentioned method for measuringthe amount of salt. This device comprises:

a first sample holding space for holding urine, which is a sample, inthe absence of picric acid and any enzyme responsive to creatinine;

a first sample inlet for introducing the urine into the first sampleholding space, the first sample inlet communicating with the firstsample holding space;

a creatinine quantitative reagent disposed in the first sample holdingspace, the creatinine quantitative reagent containing a metal complex ofat least one of hexacyanoferrate and hexacyanoruthenate;

two or more electrodes disposed in the first sample holding space or anoptical measurement window disposed on the first sample holding space,

a second sample holding space for holding the urine;

a second sample inlet for introducing the urine into the second sampleholding space, the second sample inlet communicating with the secondsample holding space; and

two or more electrodes disposed in the second sample holding space.

The apparatus for measuring the concentration of creatinine according tothe invention comprises:

a measuring device mounting port for mounting the above-mentioned devicefor measuring the concentration of creatinine;

a measurement system for electrochemically or optically measuring theamount of the metal complex reduced by the creatinine in the sampleholding space of the measuring device; and

an arithmetic unit for determining the concentration of the creatininecontained in the sample from the amount of the reduced metal complexmeasured by the measurement system.

The apparatus for measuring the amount of salt according to theinvention comprises:

a measuring device mounting port for mounting the above-mentioned devicefor measuring the amount of salt;

a first measurement system for electrochemically or optically measuringthe amount of the metal complex reduced by the creatinine in the firstsample holding space of the measuring device;

a second measurement system for measuring an electrical property of theurine in the second sample holding space of the measuring device; and

an arithmetic unit for determining a value reflecting the amount of saltexcreted in the urine from the amount of the reduced metal complexmeasured by the first measurement system and the electrical propertymeasured by the second measurement system.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the method for measuring the concentration of creatinine ofthe invention, creatinine contained in a sample is quantified with goodaccuracy in the absence of picric acid and any enzyme responsive tocreatinine.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an exploded perspective view showing the structure of a devicefor measuring creatinine concentration in Embodiment 1 of the invention;

FIG. 2 is a perspective view showing the appearance of an apparatus formeasuring creatinine concentration in the same embodiment;

FIG. 3 is a block diagram showing the configuration of the apparatus formeasuring creatinine concentration in the same embodiment;

FIG. 4 is an exploded perspective view showing the structure of a devicefor measuring creatinine concentration in Embodiment 2 of the invention;

FIG. 5 is a perspective view showing the appearance of an apparatus formeasuring creatinine concentration in the same embodiment;

FIG. 6 is a block diagram showing the configuration of the apparatus formeasuring creatinine concentration in the same embodiment;

FIG. 7 is an exploded perspective view showing the structure of a devicefor measuring the amount of salt in Embodiment 3 of the invention seenfrom the first face side of the first substrate;

FIG. 8 is an exploded perspective view showing the structure of thedevice for measuring the amount of salt seen from the second face sideof the first substrate in the same embodiment;

FIG. 9 is a perspective view showing the appearance of an apparatus formeasuring the amount of salt in the same embodiment;

FIG. 10 is a block diagram showing the configuration of the apparatusfor measuring the amount of salt in the same embodiment;

FIG. 11 is a graph showing the relationship between the creatinineconcentration in samples and the current value measured in Example 1 ofthe invention;

FIG. 12 is a graph showing the relationship between the potentialapplied to the first electrode and the current value measured in Example4 of the invention;

FIG. 13 is a graph showing the relationship between the creatinineconcentration in samples and the current value measured in the sameExample;

FIG. 14 is a graph showing the relationship between the creatinineconcentration in samples and the current value measured in the Example 5of the invention; and

FIG. 15 is a graph showing variation in the current value measured withcreatinine concentration measuring devices of the same Example and aReference Example.

DESCRIPTION OF EMBODIMENTS

The inventors have found that creatinine directly reacts withhexacyanoferrate (III) (trivial name: ferricyanide), which is atrivalent anion represented by the following formula (4). In thisreaction, in the absence of picric acid and any enzyme responsive tocreatinine (e.g., creatinine amidohydrolase, creatinine deiminase),creatinine reacts with trivalent hexacyanoferrate to form an oxidationproduct of creatinine and hexacyanoferrate (II) (trivial name:ferrocyanide), which is a tetravalent anion represented by the followingformula (5). The inventors have found as a result of analyses thatmethylguanidine and N-methyluric acid are formed as reaction products.Therefore, the oxidation product of creatinine produced in this reactionis estimated to be creatol.

[Fe(CN)₆]³⁻  (4)

[Fe(CN)₆]⁴⁻  (5)

Also, the inventors have found that creatinine directly reacts withhexacyanoruthenate (III), which is a trivalent anion represented by thefollowing formula (6). In this reaction, in the absence of picric acidand any enzyme responsive to creatinine, creatinine reacts withtrivalent anion hexacyanoruthenate to form an oxidation product ofcreatinine and hexacyanoruthenate (II), which is a tetravalent anionrepresented by the following formula (7).

[Ru(CN)₆]³⁻  (6)

[Ru(CN)₆]⁴⁻  (7)

The invention is based on the above-noted findings and characterized inthat at least one of hexacyanoferrate and hexacyanoruthenate is used asa reagent for the measurement of creatinine concentration.

Hexacyanoferrate and hexacyanoruthenate are usually present in the formof a complex salt. For example, in the state of a solid,hexacyanoferrate or hexacyanoruthenate forms a complex salt with acounter cation. In a solution, a complex salt of hexacyanoferrate orhexacyanoruthenate is ionized, and hexacyanoferrate orhexacyanoruthenate is present in the form of a solvated anion.

The speed of the reaction between creatinine and a reagent, inparticular, creatinine and trivalent hexacyanoferrate, is accelerated inthe presence of a phosphate buffer. Also, when creatinine is reactedwith a reagent in the presence of a phosphate buffer, the presence of acationic hydrophilic polymer enhances the reproducibility of thereaction.

The method for measuring the concentration of creatinine according tothe invention includes:

(A) mixing a sample containing creatinine with a creatinine quantitativereagent containing a metal complex of at least one of hexacyanoferrateand hexacyanoruthenate in the absence of picric acid and any enzymeresponsive to creatinine, to cause the creatinine to reduce the metalcomplex;

(B) electrochemically or optically measuring the amount of the metalcomplex reduced in the step (A); and

(C) determining the concentration of the creatinine contained in thesample from the amount of the reduced metal complex measured in the step(B).

According to this method, unlike conventional measuring methods,creatinine directly reacts with the metal complex contained in thecreatinine quantitative reagent in the absence of picric acid and anyenzyme responsive to creatinine. Therefore, the reaction proceedswithout being affected by intereferents including ion species such assalt, urea, proteins, amino acids, sugars, acetone, and bilirubin.Hence, even when a biological sample such as urine or blood is used, itis possible to quantify creatinine contained in the sample with betteraccuracy than conventional measuring methods.

The creatinine quantitative reagent is preferably hexacyanoferrate.Hexacyanoferrate is chemically stable, and reacts with creatinineefficiently. Examples of usable complex salts of hexacyanoferrateinclude potassium ferricyanide and sodium ferricyanide.

Hexacyanoferrate may be, before being reacted with creatinine, trivalenthexacyanoferrate, i.e., an oxidized form of hexacyanoferrate, or may betetravalent hexacyanoferrate, i.e., a reduced form of hexacyanoferrate.When the creatinine quantitative reagent is tetravalenthexacyanoferrate, the tetravalent hexacyanoferrate dissolved in a sampleis oxidized to trivalence as appropriate. Trivalent hexacyanoferrate isobtained, for example, by oxidizing tetravalent hexacyanoferrate on anelectrode.

The creatinine quantitative reagent may be hexacyanoruthenate. Examplesof usable complex salts of hexacyanoruthenate include potassiumhexacyanoruthenate and sodium hexacyanoruthenate.

Hexacyanoruthenate may be, before being reacted with creatinine,trivalent hexacyanoruthenate, i.e., an oxidized form ofhexacyanoruthenate, or may be tetravalent hexacyanoruthenate, i.e., areduced form of hexacyanoruthenate. When the creatinine quantitativereagent is tetravalent hexacyanoruthenate, the tetravalenthexacyanoruthenate dissolved in a sample is oxidized to trivalence asappropriate. Trivalent hexacyanoruthenate is obtained, for example, byoxidizing tetravalent hexacyanoruthenate on an electrode.

In the step A, the sample may be further mixed with a buffer.

Examples of buffers include phosphate buffers such as dipotassiumhydrogen phosphate and potassium dihydrogen phosphate, citrate buffers,phthalate buffers, acetate buffers, an MES (2-Morpholinoethane sulfonicacid) buffer. It is preferable to mix such a buffer with the sample sothat the pH of the sample is adjusted to 3 to 6. It is particularlypreferable to mix a phosphate buffer with the sample so that the pH ofthe sample is adjusted to 5 to 6.

When the creatinine quantitative reagent is hexacyanoferrate, it isparticularly preferable to mix a phosphate buffer with the sample sothat the pH of the sample is adjusted to 5 to 6. This increases thespeed of the direct reaction between creatinine and trivalenthexacyanoferrate, thereby allowing a reduction in measurement time.

The phosphate buffer is preferably composed of dipotassium hydrogenphosphate and potassium dihydrogen phosphate. By dissolving thesephosphates in the sample, the pH of the sample is readily adjusted tothe range of 5 to 6.

The concentration of the phosphate buffer (concentration of phosphorusatoms) in the sample is preferably 5 to 1100 mM, and more preferably 5to 500 mM. The inventors have found that as the concentration of thephosphate ion increases, the speed of the reaction between creatinineand trivalent hexacyanoferrate increases. If the concentration of thephosphate buffer is 5 mM or more, a sufficient reaction speed isobtained. Also, 1100 mM is the upper limit of the solubility of thephosphate buffer.

In the step A, the sample may be mixed with a cationic hydrophilicpolymer in addition to the buffer. Hexacyanoferrate, hexacyanoruthenate,and the phosphate buffer are anionic. Thus, the reagent is thought to beelectrostatically drawn by the cationic group of the cationichydrophilic polymer so that the reagent becomes uniform. Probably forthis reason, creatinine contained in a sample is quantified with goodreproducibility.

The concentration of the cationic hydrophilic polymer in the sample ispreferably 0.02 to 0.5% by weight. If the concentration of the cationichydrophilic polymer is 0.02% by weight or more, reproducibility isimproved sufficiently. Also, if the concentration of the cationichydrophilic polymer is 0.5% by weight or less, the cationic hydrophilicpolymer is sufficiently dissolved in a sample.

An example of cationic hydrophilic polymers is cationic guar gum. Guargum is a polysaccharide derived from the endosperm of seeds of guar,which is a leguminous plant. Cationic guar gum is a cationized form ofguar gum. An example of cationic guar gum used in the invention is guarhydroxypropyltrimonium chloride.

In the method for measuring the concentration of creatinine according tothe invention, it is most preferable to mix a phosphate buffer and acationic hydrophilic polymer to a sample so that the pH of the sample isadjusted to the range of 5 to 6.

When the amount of the reduced metal complex is electrochemicallymeasured, for example, the step (B) comprises:

(D) bringing the sample into contact with two or more electrodes andapplying a voltage between the two electrodes; and

(E) detecting the current value or the amount of electric charge flowingbetween the two electrodes.

Also, the step (C) comprises the step of determining the concentrationof creatinine contained in the sample from the current value or theamount of electric charge detected in the step (E).

In this case, the concentration of creatinine contained in a sample iselectrochemically determined with ease.

When the amount of the reduced metal complex is optically measured, forexample, the step (B) comprises the steps of:

(F) irradiating the sample with light; and

(G) detecting the light transmitted through the sample or the lightreflected by the sample.

Also, the step (C) comprises the step of determining the concentrationof creatinine contained in the sample from the intensity of thetransmitted light or the reflected light detected in the step (G).

In this case, the concentration of creatinine contained in a sample isoptically determined with ease.

The method for measuring the amount of salt according to the inventionuses urine as a sample, and comprises the following steps (c) and (d) inaddition to the steps (A) and (B) of the above-mentioned method formeasuring creatinine concentration.

That is, the method for measuring the amount of salt according to theinvention comprises the steps of:

(a) mixing urine, which is a sample, with a creatinine quantitativereagent containing a metal complex of at least one of hexacyanoferrateand hexacyanoruthenate in the absence of picric acid and any enzymeresponsive to creatinine, to cause creatinine contained in the urine toreduce the metal complex;

(b) electrochemically or optically measuring the amount of the metalcomplex reduced in the step (a);

(c) measuring an electrical property of the urine; and

(d) determining a value reflecting the amount of salt excreted in theurine from the amount of the reduced metal complex measured in the step(b) and the electrical property measured in the step (c).

An electrical property of urine in which no creatinine quantitativereagent is dissolved reflects the concentration of electrolyte containedin the urine. The concentration of electrolyte contained in the urinecorrelates with the concentration of salt contained in the urine.Components such as salt are affected by water intake, sweating, and thelike, so they are concentrated or diluted before being excreted in theurine. Thus, the concentration of urinary components such as salt in arandom urine sample, which is a urine sample collected randomlyirrespective of daytime or nighttime, fluctuates due to the influence ofconcentration and dilution of urine.

On the other hand, the amount of creatinine produced is dependent on theamount of muscle, as described above. It is thus known that the amountof urinary creatinine excretion per unit time is constant. Even in thecase of using a random urine sample, the influence of the concentrationand dilution of urine is corrected, for example, by obtaining the ratioof the concentration of a measured urinary component to the creatinineconcentration (the ratio of urinary component/creatinine).

The method for measuring the amount of salt according to the inventionuses the value measured in the step (b), which reflects the creatinineconcentration with high accuracy and good reproducibility, and theelectrical property measured in the step (c), which reflects the saltconcentration. As a result, the influence of the concentration anddilution of urine is corrected with high accuracy and goodreproducibility. It is therefore possible to obtain a value thatproperly reflects the amount of urinary salt excretion.

Examples of electrical properties of urine include resistance,conductivity, impedance, voltage (or current) signal produced inresponse to input current (or voltage) signal, and phase differencebetween the phase of input AC signal and the phase of output AC signal.

Examples of values reflecting the amount of urinary salt excretiondetermined in the step (d) include the amount of salt per unit amount ofcreatinine, the amount of urinary salt excretion per unit time (e.g., 1day), and the amount of salt intake per unit time (e.g., 1 day).

The device for measuring the concentration of creatinine according tothe invention comprises:

a sample holding space for holding a sample containing creatinine in theabsence of picric acid and any enzyme responsive to creatinine;

a sample inlet for introducing the sample into the sample holding space,the sample inlet communicating with the sample holding space; and

a creatinine quantitative reagent disposed in the sample holding space,the creatinine quantitative reagent containing a metal complex of atleast one of hexacyanoferrate and hexacyanoruthenate. This device isused in the above-mentioned method for measuring the concentration ofcreatinine.

With this device, creatinine directly reacts with the metal complexcontained in the reagent in the sample holding space in the absence ofpicric acid and any enzyme responsive to creatinine, unlike conventionalmeasuring devices. Therefore, the reaction proceeds without beingaffected by interferents including ion species such as salt, urea,proteins, amino acids, sugars, acetone, and bilirubin. Hence, even inthe case of using a biological sample such as urine or blood, it ispossible to quantify creatinine contained in the sample with betteraccuracy than conventional measuring devices. Also, hexacyanoferrate,hexacyanoruthenate, and a phosphate buffer are anionic. Thus, they arethought to be electrostatically drawn by the cationic group of thecationic hydrophilic polymer so that the reagent becomes uniform.Probably for this reason, creatinine contained in a sample is quantifiedwith good reproducibility.

The device for measuring the concentration of creatinine may include aphosphate buffer in the sample holding space, or may further include acationic hydrophilic polymer.

The device for measuring the concentration of creatinine may include twoor more electrodes in the sample holding space, or an opticalmeasurement window disposed on the sample holding space.

In this case, the concentration of creatinine contained in a sample iselectrochemically or optically determined with ease.

The device for measuring the amount of salt according to the inventioncomprises:

a first sample holding space for holding urine, which is a sample, inthe absence of picric acid and any enzyme responsive to creatinine;

a first sample inlet for introducing the urine into the first sampleholding space, the first sample inlet communicating with the firstsample holding space;

a creatinine quantitative reagent disposed in the first sample holdingspace, the creatinine quantitative reagent containing a metal complex ofat least one of hexacyanoferrate and hexacyanoruthenate;

a second sample holding space for holding the urine;

a second sample inlet for introducing the urine into the second sampleholding space, the second sample inlet communicating with the secondsample holding space; and

two or more electrodes disposed in the second sample holding space. Thisdevice is used in the above-mentioned method for measuring the amount ofsalt.

This device can efficiently measure the amount of reduced metal complex,which accurately reflects the concentration of creatinine, and anelectrical property of urine, which reflects the concentration of salt.By using the amount of reduced metal complex and the electricalproperty, the influence of concentration and dilution of urine iscorrected with good accuracy. It is thus possible to obtain a valuewhich properly reflects the amount of urinary salt excretion.

The device for measuring the amount of salt may further include two ormore electrodes in the first sample holding space, or an opticalmeasurement window disposed on the sample holding space.

The apparatus for measuring the concentration of creatinine according tothe invention comprises:

a measuring device mounting port for mounting the above-mentioned devicefor measuring the concentration of creatinine;

a measurement system for electrochemically or optically measuring theamount of the metal complex reduced by the creatinine in the sampleholding space of the measuring device; and

an arithmetic unit for determining the concentration of the creatininecontained in the sample from the amount of the reduced reagent measuredby the measurement system.

This measuring apparatus can electrochemically or optically measure theconcentration of creatinine contained in a sample by using theabove-mentioned device for measuring the concentration of creatinine.

When the concentration of creatinine contained in a sample is opticallymeasured, the measurement system includes, for example, a light sourcefor emitting light to the sample holding space of the measuring device,and a light receiver for detecting the light transmitted through thesample holding space or the light reflected in the sample holding space.Also, the arithmetic unit determines the concentration of the creatininecontained in the sample from the intensity of the transmitted light orthe reflected light detected by the light receiver.

When the device for measuring the concentration of creatinine furtherincludes two or more electrodes in the sample holding space, themeasurement system includes, for example, a voltage application unit forapplying a voltage between the two electrodes, and a detector fordetecting the current value or the amount of electric charge flowingbetween the two electrodes. Also, the arithmetic unit determines theconcentration of the creatinine contained in the sample from the currentvalue or the amount of electric charge detected by the detector.

The apparatus for measuring the amount of salt according to theinvention comprises:

a measuring device mounting port for mounting the above-mentioned devicefor measuring the amount of salt;

a first measurement system for electrochemically or optically measuringthe amount of the metal complex reduced by the creatinine in the firstsample holding space of the measuring device;

a second measurement system for measuring an electrical property of theurine in the second sample holding space of the measuring device; and

an arithmetic unit for determining a value reflecting the amount of saltexcreted in the urine from the amount of the reduced metal complexmeasured by the first measurement system and the electrical propertymeasured by the second measurement system.

This measuring apparatus can correct the thickness of urine based on theurinary creatinine concentration quantified with high accuracy and goodreproducibility and the measured electrical property of the urine. It istherefore possible to determine the amount of urinary salt with highaccuracy and good reproducibility.

Examples of samples include aqueous solutions and body fluids such asblood, blood serum, blood plasma, urine, interstitial fluid, lymph, andsaliva. In particular, urine is a very effective sample fornon-invasive, daily healthcare at home. Since the concentration of ionspecies and urea in these body fluids is relatively high, the inventionis very effective.

Preferable electrode materials used in the invention include at leastone of gold, platinum, palladium, alloys and mixtures thereof, andcarbon. These materials are chemically and electrochemically stable,thus realizing stable measurements. As a third electrode, it is alsopossible to use an electrode with stable potential, for example, areference electrode such as an Ag/AgCl or saturated calomel electrode,in combination with the above-mentioned two electrodes. If the potentialof one of the two electrodes is regulated relative to the thirdelectrode, the potential for measurement becomes stable, which ispreferable. Also, as the other electrode of the two electrodes, forexample, an Ag/AgCl or saturated calomel electrode is used.

In the measuring devices of the invention, it is preferable that thecreatinine quantitative reagent be stored in a dry state and dissolvedby a sample when the sample is introduced in the sample holding space.

In the measuring devices of the invention, it is preferable that thebuffer and the cationic hydrophilic polymer be stored in a dry state anddissolved by a sample when the sample is introduced in the sampleholding space.

For example, a porous carrier made of glass fibers, filter paper, or thelike is impregnated with a solution containing a creatinine quantitativereagent, and dried to dispose the creatinine quantitative reagent on thecarrier. The carrier is then disposed on a portion to come into contactwith a sample. Also, a solution containing a creatinine quantitativereagent may be directly applied to a portion of a wall of a measuringdevice to come into contact with a sample, and dried to dispose thecreatinine quantitative reagent thereon. The solution containing acreatinine quantitative reagent may include a buffer and a cationichydrophilic polymer.

It is preferable that the above-described devices be detachably mountedin the measuring device mounting ports of the measuring apparatuses.Also, in the case of using biological liquids such as urine and blood,in particular, it is preferable for hygienic reasons that the measuringdevices be disposable.

Embodiments of the invention are hereinafter described with reference todrawings.

Embodiment 1

A device 100 for measuring creatinine concentration according toEmbodiment 1 of the invention is described with reference to FIG. 1.FIG. 1 is an exploded perspective view showing the structure of themeasuring device 100.

The measuring device 100 is used in a method for electrochemicallyquantifying the concentration of creatinine contained in a sample. Themeasuring device 100 is composed of an insulating first substrate 102and an insulating second substrate 104 with an air vent 108 which arecombined so as to sandwich an insulating spacer 106 with a slit 110. Thefirst substrate 102, the second substrate 104, and the spacer 106 aremade of, for example, polyethylene terephthalate.

The first substrate 102 has a first electrode 112, a second electrode114, a first lead 122 electrically connected to the first electrode 112,and a second lead 124 electrically connected to the second electrode114. Formed on the first electrode 112 and the second electrode 114 is areagent layer 130 containing a creatinine quantitative reagent. Thedimensions of the first substrate 102 may be suitably set; for example,the width is approximately 7 mm, the length is approximately 30 mm, andthe thickness is approximately 0.7 mm.

Next, the method for producing the measuring device 100 is described. Inthis embodiment, potassium ferricyanide, which is a complex salt ofhexacyanoferrate, is used as the creatinine quantitative reagent.

First, palladium is sputtered onto the first substrate 102 with a resinmask of an electrode pattern thereon, to form the first electrode 112,the second electrode 114, the first lead 122, and the second lead 124.The first electrode 112 and the second electrode 114 are electricallyconnected to the terminals of an apparatus for measuring creatinineconcentration, which will be described below, by the first lead 122 andthe second lead 124, respectively.

Next, a given amount of an aqueous solution of potassium ferricyanide,potassium dihydrogen phosphate, and dipotassium hydrogen phosphate or anaqueous solution of potassium ferricyanide, cationic guar gum, potassiumdihydrogen phosphate, and dipotassium hydrogen phosphate is dropped onthe first electrode 112 and the second electrode 114 formed on the firstsubstrate 102 with a microsyringe or the like. Thereafter, the firstsubstrate 102 is left for drying in an environment at room temperatureto approximately 30° C., to form the reagent layer 130.

The concentration and amount of the reagent-containing aqueous solutionto be applied thereto are selected depending on the characteristics andsize of the necessary device. For example, the concentration of thetrivalent hexacyanoferrate in the reagent-containing aqueous solution isapproximately 0.1 M, and the dropping amount of the aqueous solution isapproximately 1.4 μL. Also, when the reagent containing aqueous solutioncontains cationic guar gum, the concentration of the cationic guar gumin the aqueous solution is approximately 0.25% by weight, and thedropping amount is approximately 1.4 μL.

The area of the region on which the reagent layer 130 is formed issuitably selected in view of the solubility of the reagent in the sampleand the like, and the area is, for example, approximately 3 mm².

Next, the first substrate 102 with the electrodes and the reagent layer130 formed thereon is combined with the spacer 106 and the secondsubstrate 104. Adhesive is applied to the portions of the firstsubstrate 102, the spacer 106, and the second substrate 104 to bebonded. They are laminated, pressed, and allowed to stand for bonding.Instead of this method, it is also possible to combine them withoutapplying adhesive and then thermally or ultrasonically bond the bondingportions by using a commercially available welding machine.

When the first substrate 102, the spacer 106, and the second substrate104 are combined, a space is formed by the slit 110 of the spacer 106between the first substrate 102 and the second substrate 104, and thisspace serves as a sample holding space. Also, the opening of the slit110 serves as a sample inlet 132.

Next, an apparatus 200 for measuring creatinine concentration accordingto this embodiment and the method for measuring creatinine concentrationusing this apparatus are described with reference to FIGS. 2 and 3. FIG.2 is a perspective view showing the appearance of the measuringapparatus 200, and FIG. 3 is a block diagram showing the configurationof the measuring apparatus 200.

First, the structure of the measuring apparatus 200 is described withreference to FIG. 2.

A housing 202 of the measuring apparatus 200 has a measuring devicemounting port 208 for mounting the measuring device 100, a display 204for displaying measurement results etc., and a measurement start button206 for starting the measurement of creatinine concentration by themeasuring apparatus 200. Inside the measuring device mounting port 208are a first terminal and a second terminal, which are to be electricallyconnected to the first lead 122 and the second lead 124 of the measuringdevice 100, respectively.

Next, the configuration inside the housing 202 of the measuringapparatus 200 is described with reference to FIG. 3.

The housing 202 of the measuring apparatus 200 contains a voltageapplication unit 302, an electrical signal detector 304, a controller306, a time measuring unit 308, and a storage unit 310.

The voltage application unit 302 has the function of applying a voltageor potential to the first electrode 112 and the second electrode 114 ofthe measuring device 100 mounted in the measuring device mounting port208. The voltage or potential is applied through the first terminal andthe second terminal electrically connected to the first lead 122 and thesecond lead 124 of the measuring device 100, respectively.

The electrical signal detector 304 has the function of detecting theelectrical signal from the first electrode 112 and the second electrode114 through the first terminal and the second terminal. The electricalsignal detector 304 corresponds to the detector of the invention.

The storage unit 310 stores correlation data corresponding to acalibration curve which indicates a correlation between creatinineconcentrations and electrical signals detected by the electrical signaldetector 304. Examples of the storage unit 310 include memory such asRAM and ROM.

The controller 306 has the function of converting the electrical signaldetected by the electrical signal detector 304 to creatinineconcentration by referring to the correlation data. The controller 306corresponds to the arithmetic unit of the invention. Examples of thecontroller 306 include microcomputers such as a CPU (Central ProcessingUnit).

Next, the method for measuring creatinine concentration using themeasuring device 100 and the measuring apparatus 200 according to thisembodiment is described.

First, a user inserts the lead side of the measuring device 100 into themeasuring device mounting port 208 of the measuring apparatus 200. As aresult, the first lead 122 and the second lead 124 of the measuringdevice 100 come into contact with and are electrically connected to thefirst terminal and the second terminal inside the measuring devicemounting port 208, respectively.

When the measuring device 100 is inserted into the measuring devicemounting port 208, an insertion detecting switch is turned on, so that asignal is sent to the controller 306. The insertion detecting switchcomprises a microswitch installed in the measuring device mounting port208. When the controller 306 detects the insertion of the measuringdevice 100 from the signal sent from the insertion detecting switch, thecontroller 306 controls the voltage application unit 302, so that avoltage (e.g., 0.2 V) is applied between the first electrode 112 and thesecond electrode 114 through the first terminal and the second terminalin order to detect the introduction of a sample.

Next, the user brings a sample into contact with the sample inlet 132 ofthe measuring device 100. Upon the contact, the sample (e.g.,approximately 0.6 μL) is sucked into the sample holding space of themeasuring device 100 from the sample inlet 132 by capillarity, so thatthe sample holding space is filled with the sample. When the samplecomes into contact with the first electrode 112 and the second electrode114, a current flows between the first electrode 112 and the secondelectrode 114 through the sample. The resultant change in electricalsignal is detected by the electrical signal detector 304.

When the controller 306 detects the introduction of the sample into thesample holding space from the signal sent from the electrical signaldetector 304, the controller 306 controls the voltage application unit302, so that the voltage applied by the voltage application unit 302 ischanged to a different voltage (e.g., 0 V or open circuit). Also, uponthe detection of introduction of the sample, the controller 306 causesthe time measuring unit 308, which is a timer, to start measuring time.

When the sample comes into contact with the reagent layer 130 exposed inthe sample holding space, potassium ferricyanide contained in thereagent layer 130 dissolves in the sample. The dissolution of potassiumferricyanide in the sample produces trivalent hexacyanoferrate. Theproduced trivalent hexacyanoferrate directly reacts with creatininecontained in the sample to form an oxidation product of creatinine andtetravalent hexacyanoferrate.

When the controller 306 determines from the signal sent from the timemeasuring unit 308 that a predetermined time (e.g., 60 seconds) haspassed, the controller 306 controls the voltage application unit 302, sothat a voltage is applied between the first electrode 112 and the secondelectrode 114 in order to measure the concentration of tetravalenthexacyanoferrate. For example, a voltage is applied so as to make thefirst electrode 112 +0.5 to +0.6 V relative to the second electrode 114.After a certain time (e.g., five seconds) from the voltage application,an electrical signal such as the current flowing between the firstelectrode 112 and the second electrode 114 is measured by the electricalsignal detector 304. At this time, the tetravalent hexacyanoferrate isoxidized at the first electrode 112. Therefore, the electrical signalmeasured by the electrical signal detector 304 is dependent on thecreatinine concentration in the sample.

The controller 306 reads the correlation data which is stored in thestorage unit 310 and which indicates a correlation between electricalsignals and creatinine concentrations and refers to it. As a result, theelectrical signal detected by the electrical signal detector 304 isconverted to the creatinine concentration in the sample.

The creatinine concentration thus determined is displayed on the display204. Upon the display of the creatinine concentration on the display204, the user can recognize that the measurement of the creatinineconcentration has been completed. It is preferred to store thecreatinine concentration thus obtained in the storage unit 310 togetherwith the time measured by the time measuring unit 308.

According to the measuring device 100, unlike conventional measuringdevices, creatinine directly reacts with trivalent hexacyanoferrate inthe sample holding space in the absence of picric acid and any enzymeresponsive to creatinine. Therefore, the reaction proceeds without beingaffected by interferents including ion species such as salt, urea,proteins, amino acids, sugars, acetone, and bilirubin. Hence, even inthe case of using a biological sample such as urine or blood, it ispossible to quantify creatinine contained in the sample with betteraccuracy than conventional measuring devices. Also, sincehexacyanoferrate and the phosphate buffer are anionic, they are thoughtto be electrostatically drawn by the cationic group of the cationichydrophilic polymer to form a uniform reagent. Probably for this reason,creatinine contained in a sample is quantified with goodreproducibility.

This embodiment has shown an example in which hexacyanoferrate is usedas the creatinine quantitative reagent, but hexacyanoruthenate may alsobe used instead. When hexacyanoruthenate is used as the creatininequantitative reagent, it is also possible to quantify creatininecontained in a sample with better accuracy than conventional measuringdevices without being affected by interferents including ions speciessuch as salt, urea, amino acids, and sugars.

This embodiment has shown an example in which the measuring device hasone reagent layer, but this is not to be construed as limiting. Themeasuring device may have two reagent layers, for example, a firstreagent layer containing a creatinine quantitative reagent and a secondreagent layer containing a phosphate buffer.

This embodiment has shown an example in which when the controllerdetects the introduction of a sample into the sample holding space, thevoltage applied by the voltage application unit is changed to adifferent voltage, but this is not to be construed as limiting. Thevoltage applied does not always need to be changed as long as a currentdependent on the creatinine concentration is obtained. It is alsopossible to apply a voltage necessary for a measurement (e.g., such avoltage that the first electrode is +0.5 V to +0.6 V relative to thesecond electrode) from the detection of insertion of the measuringdevice and continue to apply that voltage after the detection ofintroduction of a sample.

This embodiment has shown an example in which the potential applied tothe first electrode to obtain the electrical signal corresponding to thetetravalent hexacyanoferrate concentration is 0.5 to 0.6 V relative tothe second electrode, but this is not to be construed as limiting. Thevoltage between the first electrode and the second electrode may be anyvoltage at which the metal complex included in the creatininequantitative reagent and reduced in the redox reaction with creatinine(tetravalent hexacyanoferrate in this embodiment) is oxidized.

This embodiment has shown an example in which the time (reaction time)from the detection of introduction of a sample to the detection of anelectrical signal is 60 seconds, but the time does not always need to bethat value. The reaction time may be shorter than the above-mentionedtime if the difference in current value corresponding to the differencein creatinine concentration is effectively detected. If the reactiontime is made longer, the reaction between creatinine and trivalent anionhexacyanoferrate is more likely to reach a complete or steady state.Hence, the amount of creatinine is quantified more accurately withoutbeing affected by ambient conditions such as temperature.

This embodiment has shown an example in which an electrical signal isdetected five seconds after the application of a potential to theelectrodes, but this time is not to be construed as limiting. This timemay be any time when the difference in electrical signal correspondingto the difference in creatinine concentration is effectively detected.

Also, the shape, number, layout, etc. of the electrode system, leads,and terminals are not to be construed as being limited to those of thisembodiment. This also applies to the other embodiments.

This embodiment has shown an example in which the amount of reducedmetal complex is measured, but the decreased amount of oxidized metalcomplex may be measured to indirectly obtain the amount of reduced metalcomplex.

In order to facilitate the introduction of a sample into the sampleholding space of the measuring device, a lecithin layer may be formed bydissolving lecithin in toluene or another organic solvent to prepare asolution, applying the solution onto the inner wall of the secondsubstrate, and drying it. With this structure, the sample amount is madeconstant with better reproducibility. It is thus possible to quantifycreatinine contained in a sample with better accuracy.

The apparatus for measuring creatinine concentration may further includea recorder for recording measurement results in a storage medium such asan SD card. When measurement results are stored in a removable storagemedium, the measurement results can be readily taken out of themeasuring apparatus. It is thus easy to have the measurement resultsanalyzed by an analytical laboratory.

The measuring apparatus may further include a transmitter fortransmitting measurement results to outside of the measuring apparatus.In this case, the measurement results may be transmitted to ananalytical department in a hospital, an analytical laboratory, or thelike. Hence, the time from the measurement to the analysis is shortened.

The measuring apparatus may further include a receiver for receiving theresults of analysis by an analytical department, an analyticallaboratory, or the like. This permits prompt feedback of the analysisresults to the user.

Embodiment 2

Next, a device 400 for measuring creatinine concentration according toEmbodiment 2 of the invention is described with reference to FIG. 4.FIG. 4 is an exploded perspective view showing the structure of themeasuring device 400.

The measuring device 400 is used in a method for optically quantifyingthe concentration of creatinine contained in a sample. The measuringdevice 400 is composed of a first substrate 102 and a second substrate104 with an air vent 108 which are combined so as to sandwich a spacer106 with a slit 110. The first substrate 102, the second substrate 104,and the spacer 106 are made of, for example, polyethylene terephthalate.

In the measuring device 400, unlike the measuring device 100 accordingto Embodiment 1, the first substrate 102 does not have a first electrode112, a second electrode 114, a first lead 122, and a second lead 124.Also, a reagent layer 130 containing a creatinine quantitative reagentis disposed on the first substrate 102, not on the first electrode 112and the second electrode 114.

Next, the method for producing the measuring device 400 is described.

First, a given amount of an aqueous solution containing a creatininequantitative reagent, which is the same as that of Embodiment 1, isdropped on the first substrate 102 by using a microsyringe or the like.Thereafter, the first substrate 102 is left for drying in an environmentat room temperature to approximately 30° C., to form the reagent layer130. The concentration and amount of the reagent-containing aqueoussolution to be applied thereto is selected depending on thecharacteristics and size of the necessary device; for example, they maybe selected in the same manner as in Embodiment 1.

Next, the first substrate 102 with the reagent layer 130 formed thereonis combined with the spacer 106 and the second substrate 104. Adhesiveis applied to the portions of the first substrate 102, the spacer 106,and the second substrate 104 to be bonded. They are laminated, pressed,and allowed to stand for bonding. Instead of this method, it is alsopossible to combine them without applying adhesive and then thermally orultrasonically bond the bonding portions by using a commerciallyavailable welding machine.

When the first substrate 102, the spacer 106, and the second substrate104 are combined, a space is formed by the slit 110 of the spacer 106between the first substrate 102 and the second substrate 104, and thisspace serves as a sample holding space. Also, the opening of the slit110 serves as a sample inlet 132.

Next, an apparatus 500 for measuring creatinine concentration accordingto this embodiment and the method for measuring creatinine concentrationusing this apparatus are described with reference to FIGS. 5 and 6. FIG.5 is a perspective view showing the appearance of the measuringapparatus 500, and FIG. 6 is a block diagram showing the configurationof the measuring apparatus 500.

First, the structure of the measuring apparatus 500 is described withreference to FIG. 5.

A housing 202 of the measuring apparatus 500 has a measuring devicemounting port 208 for mounting the measuring device 400, a display 204for displaying measurement results etc., and a measurement start button206 for starting the measurement of creatinine concentration by themeasuring apparatus 500.

Next, the configuration inside the housing 202 of the measuringapparatus 500 is described with reference to FIG. 6.

The housing 202 of the measuring apparatus 500 contains a light source502, a light receiver 504, a controller 306, a time measuring unit 308,and a storage unit 310.

The light source 502 has the function of emitting light to the sampleholding space of the measuring device 400 mounted in the measuringdevice mounting port 208. The wavelength of the light emitted from thelight source 502 is selected such that the absorption intensity changesdepending on the reaction between creatinine and the metal complex inthe creatinine quantitative reagent.

The light receiver 504 has the function of detecting the light emittedfrom the light source 502 and reflected in the sample holding space ofthe measuring device 400 mounted in the measuring device mounting port208.

The storage unit 310 stores correlation data corresponding to acalibration curve which indicates a correlation between creatinineconcentrations and reflected light intensities detected by the lightreceiver 504. Examples of the storage unit 310 include memory such asRAM and ROM.

The controller 306 has the function of converting the intensity of thereflected light detected by the light receiver 504 to creatinineconcentration by referring to the correlation data. The controller 306corresponds to the arithmetic unit of the invention. Examples of thecontroller 306 include microcomputers such as a CPU (Central ProcessingUnit).

Next, the method for measuring creatinine concentration using themeasuring device 400 and the measuring apparatus 500 according to thisembodiment is described.

First, a user inserts the other side of the measuring device 400 fromthe sample inlet 132 into the measuring device mounting port 208 of themeasuring apparatus 500.

When the measuring device 400 is inserted into the measuring devicemounting port 208, an insertion detecting switch is turned on, so that asignal is sent to the controller 306. The insertion detecting switchcomprises a microswitch installed in the measuring device mounting port208. When the controller 306 detects the insertion of the measuringdevice 400 from the signal sent from the insertion detecting switch, thecontroller 306 actuates the light source 502. As a result, light isemitted to the sample holding space of the measuring device 400 from thelight source 502.

Next, the user brings a sample into contact with the sample inlet 132 ofthe measuring device 400. Upon the contact, the sample is sucked intothe sample holding space of the measuring device 400 from the sampleinlet 132 by capillarity, so that the sample holding space is filledwith the sample. When the sample reaches the position of the sampleholding space to which the light is emitted, the transmittance insidethe sample holding space changes. The resulting change in the intensityof reflected light is detected by the light receiver 504.

When the controller 306 detects the introduction of the sample into thesample holding space from the signal sent from the light receiver 504,the controller 306 causes the time measuring unit 308, which is a timer,to start measuring time.

When the sample comes into contact with the reagent layer 130 exposed inthe sample holding space, potassium ferricyanide contained in thereagent layer 130 dissolves in the sample. The dissolution of potassiumferricyanide in the sample produces trivalent hexacyanoferrate. Theproduced trivalent hexacyanoferrate directly reacts with creatininecontained in the sample to form an oxidation product of creatinine andtetravalent hexacyanoferrate. The change of the trivalenthexacyanoferrate to the tetravalent hexacyanoferrate causes a change inthe absorption spectrum of the sample. The amount of change of theabsorption spectrum of the sample is dependent on the concentration ofthe produced tetravalent hexacyanoferrate.

When the controller 306 determines from the signal sent from the timemeasuring unit 308 that a predetermined time (e.g., 60 seconds) haspassed, it causes the light receiver 504 to measure the intensity of thelight reflected in the sample holding space. The intensity of thereflected light measured by the light receiver 504 is dependent on theconcentration of creatinine contained in the sample.

The controller 306 reads the correlation data which is stored in thestorage unit 310 and which corresponds to a calibration curve indicatinga correlation between creatinine concentrations and reflected lightintensities detected by the light receiver 504, and refers to it. As aresult, the intensity of the reflected light detected by the lightreceiver 504 is converted to the creatinine concentration in the sample.

The creatinine concentration thus determined is displayed on the display204. Upon the display of the creatinine concentration on the display204, the user can recognize that the measurement has been completed. Itis preferred to store the creatinine concentration thus obtained in thestorage unit 310 together with the time measured by the time measuringunit 308.

According to the measuring device 400, unlike conventional measuringdevices, creatinine directly reacts with trivalent hexacyanoferrate inthe sample holding space in the absence of picric acid and any enzymeresponsive to creatinine. Therefore, the reaction proceeds without beingaffected by interferents including ion species such as salt, urea,proteins, amino acids, sugars, acetone, and bilirubin. Therefore, evenin the case of using a biological sample such as urine or blood, it ispossible to quantify creatinine contained in the sample with betteraccuracy than conventional measuring devices. Also, sincehexacyanoferrate and the phosphate buffer are anionic, they are thoughtto be electrostatically drawn by the cationic group of the cationichydrophilic polymer to form a uniform reagent. Probably for this reason,creatinine contained in a sample is quantified with goodreproducibility.

In this embodiment, the measuring device may include two or more reagentlayers in the same manner as in Embodiment 1.

This embodiment has shown an example in which the time (reaction time)from the detection of introduction of a sample to the detection ofreflected light intensity is 60 seconds, but the time does not alwaysneed to be that value. The reaction time may be shorter than theabove-mentioned time if the difference in reflected light intensitycorresponding to the difference in creatinine concentration iseffectively detected. If the reaction time is made longer, the amount ofcreatinine is determined more accurately.

In order to facilitate the introduction of a sample into the sampleholding space, the measuring device may have a lecithin layer in thesame manner as in Embodiment 1.

In the same manner as in Embodiment 1, the apparatus for measuringcreatinine concentration may further include a recorder for recordingmeasurement results in a storage medium such as an SD card. Also, themeasuring apparatus may further include a transmitter for transmittingmeasurement results to outside of the measuring apparatus. Further, themeasuring apparatus may further include a receiver for receiving theresults of analysis by an analytical department, an analyticallaboratory, or the like.

Embodiment 3

Next, a device 700 for measuring the amount of salt according toEmbodiment 3 of the invention is described with reference to FIGS. 7 and8. FIG. 7 is an exploded perspective view showing the structure of themeasuring device 700 on the first face side of the first substrate, andFIG. 8 is an exploded perspective view showing the structure on thesecond face side of the first substrate.

The measuring device 700 is used in a method of electrochemicallymeasuring creatinine contained in urine, i.e., sample, and measuring anelectrical property of the urine in order to estimate the amount ofurinary salt excretion in a day from the results of these measurements.

In the measuring device 700, a first face 702 of an insulating firstsubstrate 102 is in contact with an insulating first spacer 106 with aslit 110, and the first substrate 102 is combined with a secondsubstrate 104 with an air vent 108 so as to sandwich the first spacer106. Further, a second face 802 of the first substrate 102 is in contactwith an insulating second spacer 706 with a slit 710, and the firstsubstrate 102 is combined with a third substrate 704 with an air vent708 so as to sandwich the second spacer 706. The first substrate 102,the first spacer 106, the second substrate 104, the second spacer 706,and the third substrate 704 are made of, for example, polyethyleneterephthalate.

The first substrate 102 has, on the first face 702, a first electrode112, a second electrode 114, a first lead 122 electrically connected tothe first electrode 112, and a second lead 124 electrically connected tothe second electrode 114, as in the measuring device 100 ofEmbodiment 1. Disposed on the first electrode 112 and the secondelectrode 114 is a reagent layer 130 containing a creatininequantitative reagent.

Disposed on the second face 802 of the first substrate 102 are a thirdelectrode 712, a fourth electrode 714, and a fifth electrode 716, and asixth electrode 718. Further disposed on the second face 802 are a thirdlead 722 electrically connected to the third electrode 712, a fourthlead 724 electrically connected to the fourth electrode 714, a fifthlead 726 electrically connected to the fifth electrode 716, and a sixthlead 728 electrically connected to the sixth electrode 718. Thedimensions of the first substrate 102 may be suitably set; for example,the width is approximately 7 mm, the length is approximately 30 mm, andthe thickness is approximately 0.7 mm.

Next, the method for producing the measuring device 700 is described.

First, palladium is sputtered onto the first face 702 of the firstsubstrate 102 with a resin mask of an electrode pattern thereon, to formthe first electrode 112, the second electrode 114, the first lead 122,and the second lead 124. The first electrode 112 and the secondelectrode 114 are electrically connected to the terminals of anapparatus for measuring the amount of salt, which will be describedbelow, by the first lead 122 and the second lead 124, respectively.

Next, palladium is sputtered onto the second face 802 of the firstsubstrate 102 with a mask of a different electrode pattern from that ofthe above-mentioned mask, to form the third electrode 712, the fourthelectrode 714, the fifth electrode 716, the sixth electrode 718, thethird lead 722, the fourth lead 724, the fifth lead 726, and the sixthlead 728. The third electrode 712, the fourth electrode 714, the fifthelectrode 716, and the sixth electrode 718 are electrically connected tothe terminals of the apparatus for measuring the amount of salt,described below, by the third lead 722, the fourth lead 724, the fifthlead 726, and the sixth lead 728, respectively.

Next, a given amount of an aqueous solution containing a creatininequantitative reagent, which is the same as that of Embodiment 1, isdropped on the first electrode 112 and the second electrode 114 formedon the first face 702 of the first substrate 102 by using a microsyringeor the like. Thereafter, the first substrate 102 is left for drying inan environment at room temperature to approximately 30° C., to form thereagent layer 130. The concentration and amount of thereagent-containing aqueous solution to be applied thereto is selecteddepending on the characteristics and size of the necessary device; forexample, they may be selected in the same manner as in Embodiment 1.

Subsequently, the second substrate 104, the first spacer 106, the firstsubstrate 102, the second spacer 706, and the third substrate 704 arecombined so that the first face 702 of the first substrate 102 contactsthe first spacer 106 and the second face 802 of the first substrate 102contacts the second spacer 706. Adhesive is applied to the portions ofthe respective components to be bonded, and they are laminated, pressed,and allowed to stand for bonding. Instead of this method, it is alsopossible to combine them without applying adhesive and then thermally orultrasonically bond the bonding portions by using a commerciallyavailable welding machine.

When the first substrate 102, the first spacer 106, and the secondsubstrate 104 are combined, a space is formed by the slit 110 of thefirst spacer 106 between the first substrate 102 and the secondsubstrate 104, and this space serves as a first sample holding space formeasuring creatinine concentration. Also, the opening of the slit 110serves as a first sample inlet 132.

When the first substrate 102, the second spacer 706, and the thirdsubstrate 704 are combined, a space is formed by the slit 710 of thesecond spacer 706 between the first substrate 102 and the thirdsubstrate 704, and this space serves as a second sample holding spacefor measuring an electrical property of urine. Also, the opening of theslit 710 serves as a second sample inlet 732

Next, an apparatus 900 for measuring the amount of salt according tothis embodiment and the method for measuring the amount of salt usingthis apparatus are described with reference to FIGS. 9 and 10. FIG. 9 isa perspective view showing the appearance of the measuring apparatus900, and FIG. 10 is a block diagram showing the configuration of themeasuring apparatus 900.

First, the structure of the measuring apparatus 900 is described withreference to FIG. 9.

A housing 202 of the measuring apparatus 900 has a measuring devicemounting port 208 for mounting the measuring device 700, a display 204for displaying measurement results etc., and a measurement start button206 for starting the measurement of creatinine concentration and anelectrical property of urine by the measuring apparatus 900. Inside themeasuring device mounting port 208 are a first terminal, a secondterminal, a third terminal, a fourth terminal, a fifth terminal, and asixth terminal, which are to be electrically connected to the first lead122, the second lead 124, the third lead 722, the fourth lead 724, thefifth lead 726, and the sixth lead 728 of the measuring device 700,respectively.

Next, the configuration inside the housing 202 of the measuringapparatus 900 is described with reference to FIG. 10.

The housing 202 of the measuring apparatus 900 contains a voltageapplication unit 302, an electrical signal detector 304, a constant ACpower source 902, a voltage detector 904, a controller 306, a timemeasuring unit 308, and a storage unit 310.

The voltage application unit 302 has the function of applying a voltageor potential to the first electrode 112 and the second electrode 114 ofthe measuring device 700 mounted in the measuring device mounting port208. The voltage or potential is applied through the first terminal andthe second terminal electrically connected to the first lead 122 and thesecond lead 124 of the measuring device 700, respectively.

The electrical signal detector 304 has the function of detecting theelectrical signal from the first electrode 112 and the second electrode114 through the first terminal and the second terminal. The electricalsignal detector 304 corresponds to the detector of the invention.

The constant AC power source 902 has the function of applying a constantalternating current between the third electrode 712 and the sixthelectrode 718 of the measuring device 700 mounted in the measuringdevice mounting port 208. The constant alternating current is appliedthrough the third terminal and the sixth terminal electrically connectedto the third lead 722 and the sixth lead 728 of the measuring device700, respectively. The alternating current applied has, for example, afrequency of approximately 1 kHz and a current value of approximately0.1 mA.

The voltage detector 904 has the function of detecting the voltage(effective value of alternating voltage) between the fourth electrode714 and the fifth electrode 716 through the fourth terminal and thefifth terminal.

The storage unit 310 stores:

(I) first correlation data corresponding to a first calibration curvewhich indicates a correlation between creatinine concentrations andelectrical signals detected by the electrical signal detector 304;

(ii) second correlation data corresponding to a second calibration curvewhich indicates a correlation between salt concentrations and voltagesdetected by the voltage detector 904; and

(iii) third correlation data corresponding to a third calibration curvewhich indicates a correlation between the amounts of urinary saltexcretion per day and salt concentrations corrected by creatinineconcentration.

Examples of the storage unit 310 include memory such as RAM and ROM.

The controller 306 has the functions of:

(I) converting the electrical signal detected by the electrical signaldetector 304 to creatinine concentration by referring to the firstcorrelation data;

(II) converting the voltage detected by the voltage detector 904 to saltconcentration by referring to the second correlation data;

(III) correcting the salt concentration by using the creatinineconcentration thus obtained; and

(IV) converting the corrected salt concentration to the amount ofurinary salt excretion per day by referring to the third correlationdata.

The controller 306 corresponds to the arithmetic unit of the invention.Examples of the controller 306 include microcomputers such as a CPU(Central Processing Unit).

Next, the method for measuring the amount of urinary salt using themeasuring device 700 and the measuring apparatus 900 according to thisembodiment is described.

First, a user inserts the lead side of the measuring device 700 into themeasuring device mounting port 208 of the measuring apparatus 900. As aresult, the first lead 122, the second lead 124, the third lead 722, thefourth lead 724, the fifth lead 726, and the sixth lead 728 of themeasuring device 700 are electrically connected to the first terminal,the second terminal, the third terminal, the fourth terminal, the fifthterminal, and the sixth terminal inside the measuring device mountingport 208, respectively.

When the measuring device 700 is inserted into the measuring devicemounting port 208, an insertion detecting switch is turned on, so that asignal is sent to the controller 306. The insertion detecting switchcomprises a microswitch installed in the measuring device mounting port208. When the controller 306 detects the insertion of the measuringdevice 700 from the signal sent from the insertion detecting switch, thecontroller 306 controls the voltage application unit 302, so that avoltage (e.g., 0.2 V) is applied between the first electrode 112 and thesecond electrode 114 through the first terminal and the second terminal.

Subsequently, the user brings a sample into contact with the firstsample inlet 132 and the second sample inlet 732 of the measuring device700. Due to this contact, the sample is sucked into the two sampleholding spaces of the measuring device 700 from the first sample inlet132 and the second sample inlet 732 by capillarity, so that the twosample holding spaces are filled with the sample.

When the sample comes into contact with the first electrode 112 and thesecond electrode 114 in the first sample holding space, a current flowsbetween the first electrode 112 and the second electrode 114 through thesample. The resulting change in electrical signal is detected by theelectrical signal detector 304.

From the signal sent from the electrical signal detector 304, thecontroller 306 detects the introduction of the sample into the first andsecond sample holding spaces.

When the controller 306 detects the introduction of the sample into thefirst and second sample holding spaces, the controller 306 controls thevoltage application unit 302, so that the voltage applied by the voltageapplication unit 302 is changed to a different voltage (e.g., 0 V oropen circuit). Also, upon the detection of introduction of the sample,the controller 306 causes the time measuring unit 308, which is a timer,to start measuring time.

Upon the detection of introduction of the sample into the second sampleholding space, the controller 306 controls the constant AC power source902, so that a constant alternating current (e.g., frequency 1 kHz,current value 0.1 mA) is applied between the third electrode 712 and thesixth electrode 718 through the third terminal and the sixth terminal.After a predetermined time (e.g., after five seconds) from theapplication of the alternating current, the voltage detector 904measures the voltage (effective value of alternating current) betweenthe fourth electrode 714 and the fifth electrode 716.

The controller 306 reads the second correlation data which is stored inthe storage unit 310 and which indicates a correlation between saltconcentrations and voltages detected by the voltage detector 904, andrefers to it. As a result, the voltage detected by the voltage detector904 is converted to the salt concentration in the sample. The saltconcentration thus determined is displayed on the display 204.

When the sample comes into contact with the reagent layer 130 in thefirst sample holding space, potassium ferricyanide contained in thereagent layer 130 dissolves in the sample. The dissolution of potassiumferricyanide in the sample produces trivalent hexacyanoferrate. Theproduced trivalent hexacyanoferrate directly reacts with creatininecontained in the sample to form an oxidation product of creatinine andtetravalent hexacyanoferrate.

When the controller 306 determines from the signal sent from the timemeasuring unit 308 that a predetermined time (e.g., 60 seconds) haspassed, the controller 306 controls the voltage application unit 302, sothat a different voltage is applied again between the first electrode112 and the second electrode 114 (for example, such a voltage that thefirst electrode 112 is +0.5 to +0.6 V relative to the second electrode114). After a certain time (e.g., five seconds) from the voltageapplication, an electrical signal such as the current flowing betweenthe first electrode 112 and the second electrode 114 is measured by theelectrical signal detector 304. At this time, the tetravalenthexacyanoferrate is oxidized at the first electrode 112. The electricalsignal measured by the electrical signal detector 304 is dependent onthe creatinine concentration in the sample.

The controller 306 reads the first correlation data which is stored inthe storage unit 310 and which indicates a correlation betweenelectrical signals and creatinine concentrations, and refers to it. As aresult, the electrical signal detected by the electrical signal detector304 is converted to the creatinine concentration in the sample.

Thereafter, the controller 306 corrects the salt concentration by usingthe creatinine concentration thus obtained. The controller 306 thenreads the third correlation data which is stored in the storage unit 310and which corresponds to the third calibration curve indicating acorrelation between the amounts of urinary salt excretion per day andsalt concentrations corrected by creatinine concentration, and refers toit. As a result, the corrected salt concentration is converted to theamount of urinary salt excretion per day.

The creatinine concentration and the amount of urinary salt excretionper day, determined in the above manner, are displayed on the display204. Upon the display of the creatinine concentration and the amount ofurinary salt excretion per day on the display 204, the user canrecognize that the measurement has been completed. It is preferred tostore the creatinine concentration and the amount of urinary saltexcretion per day in the storage unit 310 together with the timemeasured by the time measuring unit 308.

According to the measuring apparatus 900, based on the saltconcentration corrected by using the creatinine concentration measuredwith high accuracy, the amount of urinary salt excretion per day iscalculated. It is therefore possible to obtain the amount of urinarysalt excretion per day with high accuracy.

This embodiment has shown an example in which hexacyanoferrate is usedas the creatinine quantitative reagent, but hexacyanoruthenate may alsobe used instead. In the case of using hexacyanoruthenate, it is alsopossible to quantify creatinine contained in a sample with betteraccuracy than conventional measuring devices without being affected byinterferents including ions species such as salt, urea, amino acids, andsugars.

In this embodiment, the measuring device may include two or more reagentlayers in the same manner as in Embodiment 1.

In this embodiment, the voltage applied by the voltage application unitdoes not always need to be changed to a different voltage as long as acurrent dependent on the creatinine concentration is obtained.

In this embodiment, the voltage between the first electrode and thesecond electrode may be any voltage at which the tetravalenthexacyanoferrate is oxidized.

In this embodiment, in the same manner as in Embodiment 1, the time(reaction time) from the detection of introduction of a sample to thedetection of an electrical signal is not to be construed as limiting.

This embodiment has shown an example in which an electrical signal isdetected five seconds after the application of a voltage between thefirst electrode and the second electrode, but this time is not to beconstrued as limiting.

This embodiment has shown an example in which the storage unit storesthe first to third correlation data, but this is not to be construed aslimiting. Instead, the storage unit may store correlation dataindicating a correlation between electrical signals detected by theelectrical signal detector, voltages detected by the voltage detector,and amounts of urinary salt excretion per unit time (e.g., per day). Inthis case, there is no need to determine creatinine concentration orsalt concentration. The amount of urinary salt excretion per unit timeis directly determined from the electrical signal detected by theelectrical signal detector and the voltage detected by the voltagedetector.

In order to facilitate the introduction of a sample into the sampleholding spaces of the device for measuring the amount of salt, alecithin layer similar to that of Embodiment 1 may be formed on theinner walls of the second substrate and the third substrate.

The apparatus for measuring the amount of salt may further include arecorder for recording measurement results in a storage medium such asan SD card.

The apparatus for measuring the amount of salt may further include atransmitter for transmitting measurement results to outside of themeasuring apparatus.

The apparatus for measuring the amount of salt may further include areceiver for receiving the results of analysis by an analyticaldepartment, an analytical laboratory, or the like.

Examples Example 1

The following experiment was conducted to confirm the effect of themethod for measuring the concentration of creatinine according to theinvention. In this example, hexacyanoferrate was used as a metal complexcontained in a creatinine quantitative reagent, and potassiumferricyanide was used as a complex salt thereof.

First, an aqueous solution of 400 mM dipotassium hydrogen phosphate(available from Wako Pure Chemical Industries, Ltd.; this was also usedin the following examples and reference examples) and an aqueoussolution of 400 mM potassium dihydrogen phosphate (available from WakoPure Chemical Industries, Ltd.; this was also used in the followingexamples and reference examples) were prepared. While monitoring with apH meter, the two aqueous solutions were mixed to adjust the pH of theresultant mixed aqueous solution to 6. In this way, a 400 mM phosphatebuffer solution (pH=6) was prepared. In this buffer solution wasdissolved potassium ferricyanide at a concentration of 400 mM.

The aqueous solution thus obtained was introduced into a glass cellcontainer, and a first electrode, a second electrode, and a thirdelectrode were immersed in the aqueous solution in the cell container.The first electrode was a gold electrode (electrode area 2 mm²). Thesecond electrode was prepared by winding up a 5-cm long platinum wire ina coil. The third electrode was an Ag/AgCl (saturated KCl aqueoussolution) reference electrode. All the electrodes are commercialproducts available from BAS Inc. The connecting terminals of the firstelectrode, the second electrode, and the third electrode weresequentially connected to the connecting terminals of the workingelectrode, counter electrode, and reference electrode of anelectrochemical analyzer (ALS-660A available from ALS Co., Ltd.).

Subsequently, a small amount of a creatinine aqueous solution with aconcentration of 500 mM (available from Wako Pure Chemical Industries,Ltd.; this was also used in the following examples and referenceexamples) was added to the aqueous solution in the cell container. Theamount of the creatinine aqueous solution added was adjusted in eachmeasurement so that the concentration of creatinine contained in theaqueous solution in the cell container was a predetermined value.

Upon the addition of creatinine, time measurement was started, and 10minutes after the addition of creatinine, a potential of 0.5 V wasapplied to the first electrode relative to the third electrode. Fiveseconds after the potential application, the current value was measured.This experiment was conducted at room temperature (approximately 25°C.).

The creatinine concentration in the aqueous solution contained in thecell container was varied to 0, 1, 2, 5, 10, 20, 30, 40, and 50 mM, andmeasurements were made in the manner as described above.

FIG. 11 is a graph of the measured current values plotted as a functionof creatinine concentration. In FIG. 11, the abscissa represents theconcentration (mM) of creatinine contained in the aqueous solution inthe cell container, and the ordinate represents the measured currentvalues (μA). As is clear from FIG. 11, the current value increaseslinearly with (i.e., in proportion to) the increase in creatinineconcentration in the aqueous solution in the cell container, whichindicates a high correlation between the current values and thecreatinine concentrations. Therefore, it is understood that the methodfor measuring the concentration of creatinine according to the inventioncan provide creatinine quantification based on current values obtained.

After the measurements, the aqueous solutions of the differentcreatinine concentrations remaining in the cell container were subjectedto a column chromatography to analyze the reaction products. Theanalysis result showed that these aqueous solutions in the cellcontainer contained tetravalent hexacyanoferrate. Also, the amount oftetravalent hexacyanoferrate produced was a maximum of 4 molecules per 1molecule of creatinine. From the above analysis result, the reactions inthe method for measuring the concentration of creatinine according tothe invention may be explained as follows.

In the sample, trivalent hexacyanoferrate reacts with creatinine in thepresence of the phosphate buffer, so that it is reduced and converted totetravalent hexacyanoferrate. That is, creatinine donates electrons totrivalent hexacyanoferrate, thereby being oxidized. In this reaction, itcan be assumed that creatinine was oxidized by a maximum of 4 electrons.The first electrode is under such a potential that electrons arereceived from tetravalent hexacyanoferrate. Thus, the producedtetravalent hexacyanoferrate is electrochemically oxidized at the firstelectrode. As a result, a current flows through the first electrode. Theconcentration of tetravalent hexacyanoferrate produced in a certain timeis dependent on the creatinine concentration. Also, the oxidationcurrent of the tetravalent hexacyanoferrate is dependent on theconcentration of tetravalent hexacyanoferrate in the sample. Therefore,the current value obtained is dependent on the creatinine concentration.

Example 2

Next, the following experiment was conducted to check the pH rangepreferable in the method for measuring the concentration of creatinineaccording to the invention. Since the sample preparation method and theconfiguration of the apparatus used in the experiment and the experimentprocedure are the same as those of Example 1, the explanation thereof isomitted. However, in this example, the creatinine concentration in thesample was set to 27 mM. Also, the pH of the phosphate buffer solutionadded to the sample was varied to 2, 2.5, 3, 4, 5, 6, 7, 8, and 9, andmeasurements were made in the same manner as in Example 1.

Table 1 shows the measurement results of current values. In the pH rangeof 2.5 to 7, in particular, the range of 3 to 6, the current value issignificantly high and the current value is stable. This result showsthat in this pH range, creatinine is quantified with particularly highsensitivity and high reproducibility. Of the above-mentioned ranges, thepH 5 to 6 can be obtained by a phosphate buffer. Also, it is found thatthe speed of the reaction between creatinine and trivalenthexacyanoferrate increases in the presence of a phosphate buffer. It istherefore preferable to mix a phosphate buffer with a sample to adjustthe pH to 5 to 6. Also, such a pH is obtained by a hydrogen phosphateion and a dihydrogen phosphate ion. It is thus thought that thepreferable phosphate buffer contains, for example, a combination ofdipotassium hydrogen phosphate or disodium hydrogen phosphate andpotassium dihydrogen phosphate or sodium dihydrogen phosphate. Also, inthe pH range of 2.5 to 7, current values can also be obtained, socreatinine quantification is possible.

pH 2 2.5 3 4 5 6 7 8 9 Current 0.1 1.6 2.7 2.9 2.9 2.8 1.6 0.4 0.2 ( μA)

Example 3

Next, the following experiment was conducted to examine the influence ofcoexistent substances that may be contained in samples on the method formeasuring the concentration of creatinine according to the invention.

Examples of coexistent substances that may be contained in biologicalsamples include ion species, enzyme denaturants, products of enzymereaction of creatinine, sugars, and amino acids. Thus, as representativecoexistent substances, this example used: NaCl, which produces ionspecies when dissolved in a sample; urea, which is an enzyme denaturant;creatine, sarcosine, and glycine, which are products of enzyme reactionof creatinine; glucose, which is a sugar; and histidine, taurine,glutamine, and serine, which are amino acids.

In the same procedure as that of Example 1, a phosphate buffer solution(pH=6) with a concentration of 50 mM was prepared, and each of thecoexistent substances was dissolved in the buffer solution at apredetermined concentration. The resultant sample was measured at a roomtemperature of 25° C. Also, the sample was heated to 60° C. toaccelerate the reaction and measured at 60° C. Except for this, thesample preparation method, the configuration of the apparatus, and theexperiment procedure are the same as those of Example 1, so theexplanation thereof is omitted.

Table 2 shows that the coexistent substances used, the concentrationsthereof, and the current values measured. In Table 2, sample 1 is aphosphate buffer solution that contains no coexistent substance andcontains 3 mM of creatinine. Also, samples 2 to 11 contain therespective coexistent substances shown in Table 2 and contain nocreatinine. In Table 2, the measured current values are relative valueswith respect to the measured current value of sample 1 which was definedas 100.

Table 2 indicates that all the samples containing the differentcoexistent substances did not exhibit an effective current, comparedwith sample 1 containing creatinine and containing no coexistentsubstance. This result has demonstrated that the method for measuringthe concentration of creatinine according to the invention is notaffected even if any of NaCl, urea, creatine, sarcosine, glycine,glucose, histidine, taurine, glutamine, and serine is contained in asample.

TABLE 2 Coexistent Concentration Current substance (mM) 25° C. 60° C.Sample 1 None — 100.0 100.0 Sample 2 NaCl 500 <0.5 — Sample 3 Urea 1000<0.5 — Sample 4 Creatine 3 <0.1 <0.1 Sample 5 Sarcosine 3 <0.1 <0.1Sample 6 Glycine 3 <0.1 <0.1 Sample 7 Glucose 10 <0.1 <0.1 Sample 8Histidine 3 <0.1 <0.1 Sample 9 Taurine 3 <0.1 <0.1 Sample 10 Glutamine 3<0.1 <0.1 Sample 11 Serine 3 <0.1 <0.1

On the other hand, it is known that in conventional methods in whichcreatinine is quantified in an alkaline solution with the use of anoxidant, such as the Jaffe method, the influence of sugars and aminoacids on measurement results is large. This is because in an alkalisolution, picric acid easily reacts with many organic molecules just ascreatinine.

A similar experiment was conducted by a conventional enzymatic method ascomparative examples. First, a 50 mM phosphate buffer solution (pH=7)was prepared which contained 7 U/mL creatinine amidohydrolase(creatininase)(CNH-311 available from TOYOBO CO., LTD.), 10 U/mLcreatine amidinohydrolase (creatinase)(CRH-221 available from TOYOBOCO., LTD.), 5 U/mL sarcosine oxidase (SAO-351 available from TOYOBO CO.,LTD.), and 100 mM potassium ferricyanide.

A sample 12 was prepared by adding creatinine to the phosphate buffersolution at a concentration of 3 mM.

A sample 13 was prepared by adding NaCl as a coexistent substance to thesample 12 at a concentration of 0.5 M.

A sample 14 was prepared by adding urea as a coexistent substance to thesample 12 at a concentration of 1 M.

Using samples 12 to 14, an experiment was conducted in the same manneras in this Example except that these samples were heated to 40° C. toaccelerate the reaction.

Table 3 shows the results of these comparative examples. The currentvalues shown therein are relative values with respect to the currentvalue of sample 12 which was defined as 100.

As is understood from Table 3, when 0.5 M NaCl or 1 M urea was presentin the sample, the current value derived from creatinine decreasedsignificantly, compared with when there was no coexistent substance.This result is probably due to the denaturation of the enzyme protein bythe high concentration of NaCl or urea. These coexistent substances havebeen found to decrease the activity of the enzymes used in themeasurements to 25 to 80%. The decrease in the current value is probablybecause the enzyme denaturation results in decreased enzyme activity,thereby reducing the reaction which proceeds in a certain period oftime. It is thought that this tendency also exists at room temperature.

TABLE 3 Coexistent substance Current Sample 12 None 100.0 Sample 13 0.5M NaCl 89.8 Sample 14   1 M Urea 96.4

The above results have shown that the method for measuring theconcentration of creatinine according to the invention can quantifycreatinine contained in a sample with better accuracy than conventionalmeasuring methods without being affected by coexistent components suchas ion species, urea, products of enzyme reaction of creatinine, sugars,and amino acids.

Example 4

The following experiment was conducted to confirm the effect of themethod for measuring the concentration of creatinine according to theinvention.

In this example, a potassium salt of tetravalent anionhexacyanoruthenate (available from Mitsuwa Chemicals Co., Ltd.)represented by the following formula (8) was used as the creatininequantitative reagent.

K₄[Ru(CN)₆]  (8)

First, an aqueous solution of 200 mM dipotassium hydrogen phosphate andan aqueous solution of 200 mM potassium dihydrogen phosphate wereprepared. While monitoring with a pH meter, the two aqueous solutionswere mixed to adjust the pH of the resultant mixed aqueous solution to6. In this way, a 200 mM phosphate buffer solution (pH=6) was prepared.In this buffer solution was dissolved the potassium salt ofhexacyanoruthenate represented by formula (8) at a concentration of 1mM.

Since the configuration of the cell and measuring apparatus used in thisexample is the same as that of Example 1, the explanation thereof isomitted.

Next, a small amount of a creatinine aqueous solution with aconcentration of 500 mM was added to the aqueous solution in the cellcontainer. The amount of the creatinine aqueous solution added wasadjusted in each measurement so that the concentration of creatininecontained in the aqueous solution in the cell container was apredetermined value. The creatinine concentration in the aqueoussolution contained in the cell container was varied to 0, 8, 16, 32, and64 mM, and measurements were made in the following manner.

After the addition of the creatinine aqueous solution, using anelectrochemical analyzer (ALS-660A available from ALS Co., Ltd.), thepotential applied to the first electrode was swept from 0.5 V to 1 Vrelative to the third electrode and then swept from 1 V to 0.5 V. Thecurrent which flowed at this time was measured. The potential sweep ratewas set to 1 mV/second.

FIG. 12 shows the measurement results (cyclic voltammogram). In FIG. 12,curves A to E represent the measurement results for the creatinineconcentrations of 0, 8, 16, 32, and 64 mM, respectively. When creatininewas not present, the redox reaction of hexacyanoruthenate on the firstelectrode, with the redox potential being near 0.75 V, was observed.When creatinine was added, the current at potentials higher than 0.8 Vincreased significantly. The above results indicate that creatinine isoxidized by the oxidized form of hexacyanoruthenate, and that theresultant reduced form of hexacyanoruthenate is oxidized on the firstelectrode. That is, they show that the electrochemical, catalyticoxidation reaction of creatinine proceeds via hexacyanoruthenate.

The oxidation current values at 0.9 V for the respective samples withthe different creatinine concentrations were obtained from the cyclicvoltammogram shown in FIG. 12, and the differences from the oxidationcurrent value for the creatinine concentration of 0 were calculated.FIG. 13 is a graph of the oxidation current value differentials plottedas a function of creatinine concentration. As shown in FIG. 13, theoxidation current value obtained increased with the increase increatinine concentration. Therefore, it has been found that obtainingthe oxidation current value by cyclic voltammetry as in this examplepermits creatinine quantification.

In this example, the oxidation current was measured by cyclicvoltammetry, but this is not to be construed as limiting. Instead, it isalso possible, for example, to apply a constant potential of 0.9 V tothe first electrode relative to the third electrode and measure theoxidation current flowing after a certain time (e.g., 3 minutes) fromthe addition of creatinine. In this case, also, the oxidation currentvalue increases depending on the creatinine concentration in the sample,so creatinine quantification is possible.

Example 5

The following experiment was conducted to confirm the effect of themethod for measuring the concentration of creatinine according to theinvention.

In this example, potassium ferricyanide was used as a metal complex ofhexacyanoferrate, and cationic guar gum was used as a cationichydrophilic polymer. Also, in the same procedure as that of Embodiment1, a device for measuring the concentration of creatinine having astructure shown in FIG. 1 was produced. In this example, commerciallyavailable guar hydroxypropyltrimonium chloride was used as cationic guargum.

The reagent-containing aqueous solution used to form the reagent layer130 was prepared as follows. First, an aqueous solution of 400 mMdipotassium hydrogen phosphate and an aqueous solution of 400 mMpotassium dihydrogen phosphate were prepared. Subsequently, whilemonitoring with a pH meter, the two aqueous solutions were mixed toadjust the pH of the resultant mixed aqueous solution to 6. In this way,a 400 mM phosphate buffer solution (pH=6) was prepared. Lastly,potassium ferricyanide was dissolved in this buffer solution at aconcentration of 100 mM, and cationic guar gum was dissolved at aconcentration of 0.25% by weight. In this way, the reagent-containingaqueous solution was obtained.

The aqueous solution prepared in the above manner was dropped in anamount of 1.4 μL on the first electrode 112 and the second electrode114, to form the reagent layer 130. The area of the region on which thereagent layer 130 was formed was set to 3 mm². Also, the volume of thesample holding space of the produced device for measuring theconcentration of creatinine was 0.6 μL.

Also, as a reference example, a device for measuring the concentrationof creatinine having a structure shown in FIG. 1 was produced in thesame manner as in Example 1, except that cationic guar gum was not addedto the aqueous solution used to form the reagent layer.

A terminal of an electrochemical analyzer (ALS-660A available from ALSCo., Ltd.) for the working electrode and two terminals for the counterand reference electrodes were connected to the first lead and the secondlead of the device for measuring the concentration of creatinine,respectively. Thereafter, an aqueous solution of creatinine, serving asa sample, was brought into contact with the sample inlet of the devicefor measuring the concentration of creatinine, so that 0.6 μL of thesample was introduced into the sample holding space. After 60 secondsfrom the introduction of the sample, using the electrochemical analyzer,a voltage was applied so that the first electrode was +0.6 V relative tothe second electrode. After 10 seconds from the voltage application, thecurrent flowing between the first electrode and the second electrode wasmeasured. The above experiment was conducted at room temperature(approximately 25° C.)

Samples with creatinine concentrations of 0, 10, 30, and 40 mM weremeasured in the above manner.

FIG. 14 a is graph of the current values measured with the device formeasuring the concentration of creatinine of Example 5, plotted as afunction of creatinine concentration. In FIG. 14, the abscissarepresents the concentration (mM) of creatinine contained in thesamples, and the ordinate represents the measured current values (pA).As is clear from FIG. 14, the current value obtained increases linearlywith (i.e., in proportion to) the increase in creatinine concentrationin the samples, which indicates a high correlation between the currentvalues and the creatinine concentrations.

FIG. 15 is a graph showing variation (coefficient of variation) of thecurrent values measured with the devices for measuring the concentrationof creatinine of Example 5 and the reference example. In FIG. 15, theabscissa represents the concentration (mM) of creatinine contained inthe samples, while the ordinate represents the coefficient of variation(%). Also, the white bars represent data on Example 5, while the blackbars represent data on the reference example.

As is understood from FIG. 15, in the measurements of all the sampleswith the different creatinine concentrations, the device for measuringthe concentration of creatinine of Example 5 exhibits low coefficientsof variation, compared with the reference example. This result indicatesthat the addition of cationic guar gum to the reagent layer enhances thereproducibility of measurements of creatinine concentration.

INDUSTRIAL APPLICABILITY

The invention is useful in the quantification of creatinine contained ina sample, in particular, a biological sample such as urine.

REFERENCE SIGNS LIST

-   100, 400 Device for Measuring Creatinine Concentration-   102 First Substrate-   104 Second Substrate-   106 Spacer (First Spacer)-   108, 708 Air Vent-   110, 710 Slit-   112 First Electrode-   114 Second Electrode-   122 First Lead-   124 Second Lead-   130 Reagent Layer-   132 Sample Inlet (First Sample Inlet)-   200, 500 Apparatus for Measuring Creatinine Concentration-   202 Housing-   204 Display-   206 Measurement Start Button-   208 Measuring Device Mounting Port-   302 Voltage Application Unit-   304 Electrical Signal Detector-   306 Controller-   308 Time Measuring Unit-   310 Storage Unit-   502 Light Source-   504 Light Receiver-   700 Device for Measuring the Amount of Salt-   702 First Face-   704 Third Substrate-   706 Second Spacer-   712 Third Electrode-   714 Fourth Electrode-   716 Fifth Electrode-   718 Sixth Electrode-   722 Third Lead-   724 Fourth Lead-   726 Fifth Lead-   728 Sixth Lead-   732 Second Sample Inlet-   802 Second Face-   900 Apparatus for Measuring the Amount of Salt-   902 Constant AC Power Source-   904 Voltage Detector

1-9. (canceled)
 10. A method for measuring an amount of salt, comprisingthe steps of: (a) mixing urine, which is a sample, with a creatininequantitative reagent containing a metal complex of at least one ofhexacyanoferrate and hexacyanoruthenate in the absence of picric acidand in the absence of any enzyme responsive to creatinine, to causecreatinine contained in the urine to reduce the metal complex; (b)electrochemically measuring an amount of the metal complex reduced inthe step (a); (c) measuring an electrical property of the urine; and (d)determining a value reflecting an amount of salt excreted in the urinefrom the amount of the reduced metal complex measured in the step (b)and the electrical property measured in the step (c).
 11. The method formeasuring an amount of salt in accordance with claim 10, wherein thestep (c) is performed before the step (a).
 12. The method for measuringan amount of salt in accordance with claim 10, wherein the step (c) isperformed after the step (b) and before the step (d).
 13. The method formeasuring an amount of salt in accordance with claim 10, wherein in thestep (a), the sample is further mixed with a phosphate buffer so that apH of the sample is adjusted to 5 to
 6. 14. The method for measuring anamount of salt in accordance with claim 10, wherein in the step (a), thesample is further mixed with a cationic hydrophilic polymer.
 15. Themethod for measuring an amount of salt in accordance with claim 14,wherein the cationic hydrophilic polymer is cationic guar gum. 16-24.(canceled)